Calipers are used to measure linear dimensions in various portable or hand held uses. Distances measured may be on the order of 0.001" or smaller. This order of resolution gives rise to difficulty in practical representation to a human operator. Traditionally, a "Vernier" type caliper used a gear reduction system which translated rotational motion of a gear to be represented as a linear distance displayed on a mechanical dial. However, such systems have inherent problems such as slop in the gears ("backlash")and limitations in accuracy of the machining of parts for calipers measuring dimensions less than 0.001". Although these limitations can be overcome through the use of anti-backlash devices and modern machining methods, these add cost, complexity and frailty.
More recently, calipers have incorporated optical sensing systems which use a sensor to detect radiation from a source, and use a signal generated by an optical sensor to generate an analog electrical signal. Currently, calipers using optical sensing systems are accurate to 0.0005". These devices (U.S. Pat. Nos. 4,008,523 and 4,684,257) typically use a glass plate in the shape of a rectangular strip, etched with lines that inhibit the passage of light or depositing a layer of light absorbing material in which lines of the absorbing material have been etched away.
Other calipers utilizing optical sensors (U.S. Pat. Nos. 4,034,477, 4,035,922 and 5,430,954) use a disk with a single row of etched lines which rotationally pass between a photocell that emits a light and an optical sensor, but otherwise operate similarly to the glass strip above. The rotational movement of the disk causes a continuous signal to be generated by the sensor relative to the amount of light it receives.
One problem with such a device is that if a user measures a dimension that is, say 0.001", the sensor moves, or the glass strip/wheel moves, 0.001", typically from one etch point to the next. This limits the accuracy the caliper because of the limitations imposed by costs of manufacturing. Typically, the accuracy of the prior art is no greater than 0.0005".
Because of the 1:1 measuring mentioned above, calipers require a dampening system to inhibit rapid movement of the measuring jaws. The 1:1 measuring requires the CPU to sample the sensor-generated signal at a sufficient rate to accurately detect when the signal reaches a level signifying a valid increment/decrement of a measuring count. If the user moves the jaws of the calipers too rapidly, the CPU sampling rate may not be high enough to detect when a signal level crosses a threshold. This may result in missed counts, producing erroneous output to the user. This result manifests itself when signal threshold crossings are too close in time, such that more than one signal threshold crossing occurs in a single sampling time of the CPU. A motion dampening device is typically employed to restrict the speed of movement of the measuring jaws to comply with the CPU sampling rate. These devices are well known in the art. The addition of the dampening device adds cost and mechanical complexity to the measuring device, and thus it is desirable to remove the need for dampening, as well as increasing the sampling rate.
An additional disadvantage of calipers which utilize a disk is that only a single row of slots are used. In order to determine the direction of rotation of the disk (which is also indicative of the direction of travel of the jaws of the calipers), two light signals must be sensed through the same slot. This is carried out with at least one photocell and two sensors. Once the signals are sensed, one of the signals must be offset by 90.degree.. This offset is done through a phase offset circuit, adding additional cost and complexity to the measuring device. In an alternative embodiment, prior art calipers utilizing a disk system require the use of two disks in order to determine direction of travel. This method also gives rise to additional cost and complexity. It would be desirable to use a single disk without additional phase offset circuitry and still be able to easily determine direction of rotation of the disk.
Another disadvantage of prior art calipers incorporating optical sensing systems is that each measuring device must be built as a single unit. The physical measuring jaws, the body of the entire device, and the optical sensing systems must be built as a whole. This precludes modularity between devices, restricting the use of the optical sensing system to a single caliper. It would be desirable to have a modular optical sensing system usable by a number of calipers or which can be retrofitted onto a mechanical caliper, as this would cut costs for these types of tools. Additionally, this would allow for ease of learning when the same display and modes of operation would be applicable to a range of different calipers, instead of a single device.
Further, prior art calipers which utilize optical sensing systems typically do not detect errors. An error can manifest itself when the signal input rate exceeds the CPU sampling rate, due to manufacturing defects in the etching of the glass strip or the wheel, resulting in varying etch widths. The prior art calipers use dampening systems to avoid exceeding the CPU sampling rate, but cannot compensate for defects in the etching process. Errors may also arise when the CPU sampling rate is exceeded, as discussed above. It would be desirable to incorporate an error detection system so that a motion dampening system would not be necessary and so that etch defects could be known to the system and compensated for, without a significant increase in manufacturing costs.
One problem with prior art calipers is signal jitter. Signal jitter is an effect seen when the user is not moving the jaws of the calipers. Due to vibrations, which may come from a variety of sources, a signal from a sensor oscillates about a point. This effect causes significant problems when the point at which the jitter occurs is at, or close to, a signal threshold. If a jittering signal crosses a signal threshold, the CPU may interpret each threshold crossing to be actual movement of the glass strip, or rotational movement of the wheel, and thus generate increment/decrement pulses. Prior art systems utilize motion dampening systems, as well as other mechanical devices, to overcome the effects of signal jitter, but these additional components increase cost and complexity of the measuring device, and may not be completely effective. It would be desirable to remove the need for motion dampening systems, or other mechanical components to compensate for signal jitter, while at the same time removing any adverse effects of signal jitter upon the measuring device, without adding mechanical complexity or cost to the device.